Cooperative Co(III)/Cu(II)-Catalyzed C-N/N-N Coupling of Imidates with Anthranils: Access to 1H-Indazoles via C-H Activation

Article abstract of DOI:10.1021/acs.orglett.6b01716

Cooperative cobalt- and copper-catalyzed C-H activation of imidate esters and oxidative coupling with anthranils allowed efficient synthesis of 1H-indazoles in the absence of metal oxidants. The anthranil acts as a convenient aminating reagent as well as an organic oxidant in this transformation. The copper catalyst likely functions at the stage of N-N formation.

Full text of DOI:10.1021/acs.orglett.6b01716

Letter
pubs.acs.org/OrgLett
Cooperative Co(III)/Cu(II)-Catalyzed C−N/N−N Coupling of Imidates
with Anthranils: Access to 1H‑Indazoles via C−H Activation
Lei Li,† He Wang,† Songjie Yu, Xifa Yang, and Xingwei Li*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
S
* Supporting Information
ABSTRACT: Cooperative cobalt- and copper-catalyzed C−H
activation of imidate esters and oxidative coupling with
anthranils allowed efficient synthesis of 1H-indazoles in the
absence of metal oxidants. The anthranil acts as a convenient
aminating reagent as well as an organic oxidant in this
transformation. The copper catalyst likely functions at the
stage of N−N formation.
ransition-metal-catalyzed direct C−H functionalization of
Scheme 1. Intermolecular Synthesis of 1H-Indazoles
T
arenes has been extensively employed over the past
decades as a powerful strategy in organic synthesis1 owing to
high atom- and step-economy.2,3 Despite the high activity of
various second- and third-row transition-metal catalysts,
applications of cost-effective, earth-abundant, and functionally
unique first-row metal catalysts are highly desirable. Pioneered
by Kanai and Matsunaga,4 the higher Lewis acidity and catalytic
activity of stable Cp*Co(III) complexes have allowed the
development of new catalytic systems of C−H activation. This
was made possible by the enhanced metal−ligand corporation
in the catalytic cycle. Afterward, the groups of Glorius,5
Ackermann,6 Ellman,7 Daugulis,8 Chang,9 and others10 have
made progress in cobalt-catalyzed C−H activation. These
catalytic systems can stay complementary to those enabled by
the rhodium(III) congeners in terms of substrate scope,
activity, and selectivity.
1H-Indazoles are known as an important skeleton that has
shown a wide range of pharmacological activities,11 including
anti-inflammatory, antiviral, antimicrobial, and anticancer
activities. Therefore, the development of inexpensive and
efficient methods to access 1H-indazoles in a green and step-
economic fashion is highly desirable. In this regard, the strategy
of transition-metal-catalyzed C−H bond activation/functional-
ization has exhibited significant potentials in indazole syn-
thesis.12 For example, Glorius reported a Rh(III)-catalyzed
oxidative annulation between imidates and sulfonyl azides to
deliver 1H-indzaoles.12f Subsequently, copper-catalyzed 1H-
indzaole synthesis via C−N/N−N bond formations has been
disclosed by the group of Zhu.12d We recently developed two
distinctive methods to construct indazoles via Rh(III)-catalyzed
C−H activation of imidates under both oxidative and redox-
neutral conditions (Scheme 1).12a,b While we have briefly
applied anthranil as an aminating reagent in one report, both
expensive rhodium(III) catalysts and a stoichiometric amount
of Cu(II) oxidant were necessary.12a Thus, direct access to
these synthetically useful substituted indazoles via base metal
catalysis needs further exploration. We now reported synergistic
cobalt- and copper-catalyzed synthesis of 1H-indazoles via C−
H activation and C−N/N−N bond formations, where
anthranils13 act as both an aminating reagent and an organic
oxidant.12d
We commenced our investigation with the screening of
reaction parameters for the coupling between imidates (1a) and
5-chloro-3-phenylbenzo[c]isoxazole (2a, Table 1). Using
[Cp*Co(CO)I2] (10 mol %)/AgSbF6 (20 mol %) as a catalyst,
a coupling occurred and the desired indazole product 3aa was
isolated in 60% yield in the presence of Cu(OAc)2 (2.1 equiv,
entry 1). Switching the catalyst to [Cp*Co(MeCN)3](SbF6)2
slightly improved the yield (entry 2). When the amount of 2a
was increased from 1.5 to 3.0 equiv, the yield of 3aa was
improved to 92% (entry 3), but lowering the catalyst loading
resulted in diminished yields (entries 4 and 5). We noted that
in some cases anthranil decomposed to a 2-aminobenzophe-
none in rhodium-catalyzed reactions, so we reasoned that it
might act as an organic oxidant. Indeed, the reaction proceeded
Received: June 13, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01716
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Organic Letters
Letter
a
a,b
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope of Imidates
b
entry
cat. (mol %)
x
yield (%)
c
1
[Cp*Co(Co)I2]2(10)/AgSbF6 (20)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (8)
[Cp*Co(MeCN)3](SbF6)2 (5)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
[Cp*Co(MeCN)3](SbF6)2 (10)
2.1
2.1
2.1
2.1
2.1
2.1
0.2
0.2
0.2
0.2
0
60
c
2
66
3
92
85
60
4
5
d
6
56
7
88
e
8
56
f
9
81
g
10
11
12
84
ND
ND
0.2
a
The reaction was carried out using imidate ester (0.2 mmol), 2a (0.6
a
The reaction was carried out using imidate ester 1a (0.2 mmol), 2a
mmol), the cobalt catalyst (0.02 mmol), and Cu(OAc)2 (0.04 mmol)
b
(0.6 mmol), Co catalyst, and Cu(OAc)2 in DCE (3 mL) at 100 °C
in DCE (3 mL) at 100 °C (sealed tube) under N2. Isolated yield after
b
c
(sealed tube) under N2. Isolated yield after column chromatography.
column chromatography. Reaction was performed with 7 mmol of 1a.
c
d
e
d
2a (0.3 mmol). 80 °C. Reaction was performed with 2a (0.3 mmol)
2a (0.5 mmol).
f
g
under 1 atm of O2. 2a (0.4 mmol). 2a (0.5 mmol).
a,b
Scheme 3. Scope of Anthranils in Indazole Synthesis
with comparable efficiency (88% yield) in the presence of 20
mol % of Cu(OAc)2 (entry 7), and the reduced coproduct 4a
was isolated yield (45% based on 2a and 130% based on 1a).
However, further decreasing the amount of 2a gave inferior
results even in the presence of O2 (entry 8). It was found that
3.0 equiv of 2a was the optimal loading (entries 7, 9, and 10).
Control experiments indicated that no desired reaction
occurred when either the Co(III) catalyst or Cu(OAc)2 was
omitted (entries 11 and 12).
With the optimized reaction conditions in hand, the scope
and generality with respect to imidates were next examined in
the coupling of 2a (Scheme 2). Thus, imidates bearing both
electron-donating and -withdrawing groups at the para position
of the phenyl ring all coupled in good to excellent yields (3aa−
ja). It was observed that the electronic properties of imidates
had no apparent effect on the reaction. In addition, meta-
substituted or disubstituted imidates (3ka−na) coupled in
moderate to excellent yields in high site selectivity, where C−H
functionalization occurred at the less hindered site. Introduc-
tion of an o-fluoro group was also tolerated, delivering 3oa in
44% yield. Besides the ethyl ester, other alkyl esters also
coupled smoothly in excellent yields (3pa,qa), although the
isopropyl imidate reacted with diminished efficiency (3ra).
We next examined the scope of the anthranil substrate. As
given in Scheme 3, introduction of functional groups such as
halogens (3ab,ac) and phenyl (3ad) to different positions of
the anthranil ring was compatible. In addition, introduction of a
substituted phenyl group into the 3-position of the anthranil
ring was fully tolerated (3ae−ah). However, when the
nonsubstituted anthranil was applied under the standard
conditions, the reaction became sluggish. To our delight,
further introduction of PivOH regained the coupling efficiency
(3ai), which likely facilitated the C−H activation process. In
comparison, a poor result was observed when the aminating
reagent was replaced by 2-azidobenzaldehyde, indicating the
intrinsic reactivity of anthranils. Thus, several substituted
a
The reaction was carried out using 1a (0.2 mmol), anthranil (0.6
mmol), the Co catalyst (0.02 mmol), and Cu(OAc)2 (0.04 mmol) in
DCE (3 mL) at 100 °C (sealed tube) under N2 for 20 h. Isolated
yield after column chromatography. PivOH (0.2 mmol). PivOH (0.2
mmol) and anthranil (0.5 mmol). Reaction was performed with 2-
azidobenzaldehyde.
b
c
d
e
anthranils coupled smoothly in high yields (3aj,ak) in the
presence of PivOH, although introduction of a 4-chloro group
reduced the reaction efficiency likely due to steric effects (3al).
To access a functionalized product, a derivatization reaction
was carried out (eq 1). Using TsN3 as an amidating reagent,
product 5 was isolated in moderate yield via a Ru(II)-catalyzed
C−H amidation process.14 Reduction of 3aa with NaBH4
afforded alcohol 6 in 83% yield (eq 2). Moreover, acid
B
DOI: 10.1021/acs.orglett.6b01716
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Organic Letters
Letter
Scheme 4. Proposed Mechanism
intermediate C. Migratory insertion of the Co−aryl bond into
the nitrene gives a tripodal intermediate D. Coordination of an
imidate to D and subsequent C−H activation releases the
aminated intermediate E with the regeneration of cobalt(III) A.
In a sequential copper-catalyzed cycle, coordination of
Cu(OAc)2 to E with extrusion of HOAc gives a Cu(II) species
F, which undergoes double single-electron transfer to generate
indazole 3aa and Cu(I) intermediate H. This interemdiate
could be oxidized by another molecule of 2a in the presence of
AcOH to regenerate the Cu(OAc)2. In this process, reductive
opening of the anthranil ring generates a nitrogen radical that is
trappable by BHT. However, an alternative Cu(II)−Cu(0)−
Cu(II) pathway remains possible in which intermediate F
reductively eliminates the product 3aa and HOAc to give a
Cu(0). Oxidation of Cu(0) by anthranil to Cu(II) then
regenerates the active copper catalyst.
treatment of 3aa led to deprotection of the ether12f followed by
intramolecular nucleophilic addition to give hemiaminal 7 in
44% yield (eq 3). Recycling of coproduct 4a has been briefly
explored.15 Satisfyingly, when a stoichiometric amount of
PhI(OAc)2 was used as an oxidant, the anthranil 2a was isolated
in 75% yield (eq 4).
To gain mechanistic insight into the catalytic system, two
side-by-side parallel reactions were carried out for coupling of
1a and 1a-d5 with anthranil 2a at a low conversion. A small
1
value of kH/kD = 1.4 was obtained on the basis of H NMR
analysis (eq 5), indicating that C−H bond cleavage is likely not
In summary, we have developed a synergistic Co/Cu-
catalyzed system to construct substituted 1H-indazoles from
easily available imidates and anthranils via a C−H activation
pathway where anthranils also serve as an organic oxidant. This
catalytic system tolerates a wide range of substrates with high
reaction efficiency. Meanwhile, mechanistic studies indicated
that the N−N formation likely involved nitrogen radical
species. Moreover, the reduced coproduct could be recycled to
the anthranil substrate under oxidative conditions.
ASSOCIATED CONTENT
* Supporting Information
■
S
involved in the turnover-limiting step. To explore the
intermediacy of radical species and hence the possibility of
single-electron transfer in N−N bond formation,12f the
amination reaction was performed in the presence of 2,6-di-
tert-butyl-4-methylphenol (BHT, eq 6). The reaction was
inhibited, but an amine (8) was isolated in 30% yield,
suggesting that a radical pathway is probably involved in this
reaction. To further probe the formation of N−N bond, the
Cu(OAc)2 was replaced by CuOAc for the coupling of 1a and
2a, and 3aa was isolated in 28% yield in the presence of AcOH
(eq 7). Switching to CuTc (0.2 equiv) gave rise to 3aa in 37%
yield, thus indicating that a Cu(I) species might be involved in
the N−N formation process.
On the basis of our preliminary results and relevant
reports,12a,b,d,f a plausible catalytic cycle is proposed in Scheme
4 (see the SI for alternative mechanisms). The reaction is
initiated by cyclometalation of the imidate to furnish a five-
membered metallacyclic intermediate A to which anthranil
coordinates to afford intermediate B, which undergoes
intramolecular N−O bond cleavage to deliver a nitrene
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01716.
Experimental procedures, characterization data, alter-
native mechanism, and 1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
* E-mail: xwli@dicp.ac.cn
Author Contributions
†L.L. and H.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge financial support from the Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, the NSFC
■
C
DOI: 10.1021/acs.orglett.6b01716
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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